Attempting to repair my Autotek SX5750, it goes into protection immediately after power is applied. After reading Babin Perry's DIY page, I figured I would give it a go into diagnosing the issue. After checking the power side and audio side FETs, the only FETs I could find a low ohm reading was on Ch 2 on the audio side of the board. I was getting 0.4 Ohms and 29 Ohms on the outer legs of the left and right FETs on Ch 2.

Currently I'm looking for a proper replacement FET; however, on the same channel there are two different branded FETs, one has a brand mark with an F, modeled IRF9540, the other SEC, modeled IRF540. Looking at the other audio channel FETs, they appear to be mixed as well, but not all of them, some were using identical branded FETs on the same ch. So I will just assume they are supposed to have the same specs, but the manufacturer just grabbed whatever they had in production.

Now when I searched for the datasheets for these brands, I came up with no results. So I went to digi-key's website, searched for the two models, and I get lots of different results. All with different voltage and power output specs. Can anyone give me any guidance as to which FETs I should use to replace my current deads ones? Also, should I be looking to replace the two different modeled FETs with the same part number, or are they supposed to be two different models (IRF9540 and IRF540)?

It would never have worked if the wrong transistors would have been installed. I'm assuming that no one has replaced any components between the time that it was working properly and the time when you disassembled it.

You can reuse them. Yes, use heatsink compound between the transistor and the pad. If the pad lifts from the heatsink apply compound between the sink and pad also.

The power side FETs are listed as FQP50N06, Mouser has two different models listed with slightly different specs (512-FQP50N06 and 512-FQP50N06L). The L model seems to be a newer design based on some of the wording listed on the website. Do you have any guidelines for choosing between the two?

Any tips on the best way to solder these style of FETs? The original solder is on the back side, and I would hate to have to remove the entire board just to solder these two FETs.

I tried to apply some solder to just the top, but the solder doesn't seem to want to flow into the joint, even though i applied some flux to the joint. Could it be the type of solder that I am using? Definitely open to suggestions before I go pulling the entire board.

Yes. You need to pull the board. For the terminals that have the greatest current flow (drain and source), on double-sided boards you should solder on the side of the board with the most copper, unless you get complete flow through the via.